Rectangular or box-shaped ducts are extensively used in heating and ventilating systems to distribute heated or cooled air throughout a structure. These ducts are commonly formed from different gauges of sheet metal in sections of predetermined length which are then connected to form a continuous duct for distributing air. Each section of a duct is formed by bending two pieces of sheet metal of the desired length at a 90.degree. angle, and with the Pittsbugh type of seam, one edge of each piece is formed with a longitudinally extending groove to form the female portion of the joint, while the other edge is bent over along its length to provide the male portion of the joint. The two parts are then assembled by inserting the male portion of each part into the female portion leaving an edge extending beyond the joint from the female portion, which edge must then be bent over to lock the seam. At the present time, this edge is bent over manually using either a hand hammer or a power hammer. This is not only a time consuming and tedious task, but because these ducts are metal and hollow, they amplify the sound. The noise created by hammering over the free edge to lock the seam is so noisy that some regulatory agencies have taken steps to require the worker's hearing to be protected or this method of the locking joint cannot be used.
Attempts have been made to develop an improved method of locking the Pittsburgh type joint, but such efforts have not been successful because they have all required the use of a mandrel inside of the duct. Many of these air ducts are being formed with insulation on the inside, and a mandrel simply will not work with such ducts. Therefore, the prior art has not provided any solution that will eliminate the tedious task of locking these joints to complete the assembly of the ducts, and more importantly, nothing has been developed to eliminate the noise from the assembly process.
There is therefore an obvious need for an improved way of completing the assembly of the sections of box-shaped sheet metal ducts which are assembled using Pittsburg type or similar locking seams. Any such method and apparatus must be simple to use, inexpensive, portable, and above all it must minimize if not eliminate the noise associated with the formation of these lock-type seams.